Transfer of power between two points in space without the use of wire or other physical tethering has long been a goal of humankind. This transfer of power is referred to as wireless power transfer (WPT). In near-field WPT, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire or by electric fields using capacitive coupling between electrodes. Near-field WPT may be referred to as non-radiative WPT. In far-field WPT, power is transmitted over long distances by beams of electromagnetic radiation, utilizing antennas and rectifiers. Far-field WPT may also be referred to as radiative WPT or (microwave) power beaming.
Far-field WPT may be used to transfer power from a source (transmitter) to one or more power consumers (receivers), where a “consumer” is defined as a device that utilizes the power to perform some task or that stores the power for future use. In far-field WPT, it is sufficient if the distance between the transmitting source antenna and the consumer receive antenna is such that each antenna resides in or near the radiating far-field of the other. The device associated with the reception and rectification of the transmitted power is typically referred to as a rectenna. A rectenna typically comprises a receiving antenna and a rectifier circuit. Sometimes the term “rectenna” also implies functionality associated with power management, e.g., voltage regulation.
Near-field WPT tends to be more efficient than far-field WPT. However, far-field WPT enables transfer of power over much greater distances and hence enables applications fundamentally different from those enabled by near-field WPT, e.g., space-based power transmission to terrestrial or planetary surfaces, transmission from a base to a robot, and transmission from a base to enable an RFID (radio frequency identification) tag or a battery-less sensor tag. Increases in efficiency of far-field WPT are highly desired to make such applications more practical and to enable additional applications.